Crude oil washing
(COW) is a system whereby oil tanks on a tanker are cleaned out between voyages
not with water, but with crude oil - the cargo itself. The solvent action
of the crude oil makes the cleaning process far more effective than when water
is used. (There is usually a final water rinse but the amount of water involved
is very low.) The system helps prevent pollution of the seas from operational
measures.

COW is mandatory on
new tankers under the International Convention for the Prevention of Pollution
by Ships (MARPOL 73/78).

The problem –
pollution from oil cleaning operations

Development of
Load on Top

Development of
Crude Oil Washing

Crude oil
washing regulations and specifications

The problem –
pollution from oil cleaning operations

Tankers carry their
cargo in a number of tanks or compartments within the hull of the ship.
Before the introduction of segregated ballast tanks, tanks were cleaned after
the oil was discharged and about one third of them filled with seawater so that
the ship's propeller is properly immersed and it has correct handling and
sea‑keeping characteristics. This process is known as
ballasting.

In the early days
of oil tanker operations it was a common practice to clean tanks by means of
jets spraying seawater. The jets washed the oil residues from the tank
surfaces, resulting in a mixture of oil and water which collected at the bottom
of the tank and was then pumped overboard. This naturally led to a
considerable amount of oil getting into the sea. The ballast water, which was
pumped overboard to make way for a fresh cargo of oil, was also
contaminated.

In the 1950s, there
were no alternative ways of cleaning tanks. The OILPOL Convention, adopted in
1954, tried to alleviate the pollution from this process by prohibiting the
discharge of oil or oily mixtures within 50 miles of land. This limit was
extended to 100 miles in certain areas which were regarded as being particularly
endangered.

In the late 1960s,
concern about the waste of oil and pollution caused by this process led the
industry to look for an alternative.

The result was to
become known as "load on top".

Development of Load on Top

Under load on top,
tanks were cleaned as previously using high-pressure hot-water cleaning
machines. However, instead of pumping the resulting oily mixtures overboard,
they were pumped into a special slop tank.

During the course
of the return voyage to the loading terminal this mixture separates. Oil, being
lighter than water, gradually floats to the surface leaving the denser water at
the bottom. This water is then pumped into the sea, leaving only crude oil in
the tank.

At the loading
terminal fresh crude oil is then loaded on top of it. The process had advantages
for the owner of the oil, since the oil normally lost during tank cleaning can
be saved (as much as 800 tons of oil on a large tanker), but the main
beneficiary was the environment. Some experts believe that without load on top
the amount of oil being dumped into the sea as a result of tank cleaning could
have reached more than 8 million tons a year.

Development of
Crude Oil Washing

The introduction
of load on top was a great contribution to the fight against marine pollution
but it did not completely eliminate pollution resulting from tank cleaning
operations. Although the amount and rate of discharge is carefully
regulated the process still resulted in some pollution occurring. In the late 1970s
an improvement was introduced. Instead of using water, the tank cleaning
machines used crude oil - in other words, the cargo itself.

When sprayed onto
the sediments clinging to the tank walls, the oil simply dissolved them, turning
them back into usable oil that could be pumped off with the rest of the cargo.
There was no need for slop tanks to be used since the process left virtually no
slops. The process became known as Crude Oil Washing (COW).

Crude oil washing
meant that the mixture of oil and water which led to so much operational
pollution in the past was virtually ended. At the same time, the owner is
able to discharge far more of this cargo than before, since less of it is left
clinging to the tank walls and bottoms.

Crude oil washing
regulations

Crude oil washing
was made mandatory for new tankers by the 1978 Protocol to the MARPOL
Convention.

MARPOL Annex I
Regulation 13 (6) requires every new crude oil tanker of 20,000 tons deadweight
and above to be fitted with a cargo tank cleaning system using crude oil
washing.

Regulation 13B
states that COW installation and arrangements should comply with at least all of
the provisions of the Specifications for the Design, Operation and Control of
Crude Oil Washing Systems adopted by IMO in 1978, as may be revised.

In 1999, IMO
adopted revised specifications for COW by resolution A. 897(21) Amendments to
the revised Specifications for the Design, Operation and Control of Crude Oil
Washing Systems (Resolution A.446(XI), as amended by resolution A.497(XII).